This invention relates to a multiuser receiving circuit, namely, a multiuser receiver which is for use in a CDMA (Code Division Multiple Access) system. Herein, it is to be noted that the multiuser receiver can respond to a code multiple input signal to output a plurality of demodulated signals to a plurality of users, respectively, and includes a plurality of stages.
Recent attention has been directed to a CDMA system as a multiple-access system for mobile communication systems. This is because such a CDMA system which uses a spread spectrum technique might be able to increase an accomodation capacity of subscribers.
In the CDMA system, a user signal is spread with a wide frequency bandwidth by the use of a preassigned individual code and is transmitted in a common frequency as a CDMA signal through a transmission from a transmitter. The receiving circuit can detect a desired signal by means of despreading process from the CDMA signal and the preassigned individual code.
However, the CDMA system is disadvantageous in that interference occurs due to cross correlation among spreading codes assigned to users.
Multiuser receivers have been known which remove such interference by the use of all of spreading codes assigned to all users, transmission channel characteristics, and etc.
One of the multiuser receivers has been proposed by M. K. Varanasi and B. Aashang in the article entitled "Multistage Detection in Asynchronous Code Division Multiple-Access Communications" (IEEE Trans. Commun., Vol. COM-38, No. 4, pp. 509-519, April 1990).
The receiver proposed in the above article demodulates all user signals in an initial stage and forms an interference replica for each user. Thereafter, all interference replicas except for the desired signal are subtracted from an input signal received to remove the interference. In the following stage, demodulation is made again about the desired signal by using a signal obtained by the initial stage. As a result, the user signal quality is improved as compared with the initial stage. From this fact, it is readily understood that Interference cancellation characteristic is gradually improved by repeating this process several times with a multistage structure.
Another receiver has been also described by Fukazawa, T. Sato, Kawabe, S. Sato, and Sugimoto in a document entitled "Configuration and Its Performance of an Interference Cancellation System using Pilot Signal for Radio Channel Estimation" (THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, B-II Vol. J77-B-II No. 11 November 1994). The receiver mentioned in this document has a serial structure for cancelling interference and is simple in structure. In addition, such a structure can save a replica memory by handling a decision symbol as an interference replica for each user.
However, on the demodulation process of each stage in this receiver, an interference cancellation characteristic has been lowered when a transmission channel estimation error is large. This is because a transmission channel characteristic which is estimated from a signal before channel interference cancellation is used in each stage.
Under the circumstances, proposal has been recently made about a receiver which uses transmission channel characteristics estimated at the respective stages to improve an interference cancellation characteristic.
Such a receiver has been described by Sawahashi, Miki, Ando, and Higuchi in a document entitled "Serial Canceller Using Recursive Channel Estimation by Pilot Symbols for DS (Direct Sequence)--CDMA" (THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, RF Communication Systems Technical Radio Report, RCS95-50, July 1995). This receiver employs a serial structure that successively demodulates user signals in the order of reception levels from a highest one to a lowest one and that successively performs interference cancellation.
At any rate, a conventional multiuser receiver of the CDMA system has a plurality of stages each of which forms an interference cancellation processing circuit for each user. This structure is substantially equivalent to that mentioned in the above-referenced documents.
Specifically, the multiuser receiver previously ranks reception levels of all users from a highest one to a lowest one and successively carries out demodulation and interference cancellation process from the highest level. Such a ranking operation may be carried out by using a reception signal once or by successively using the interference cancellation signal at each stage.
In case of performing interference cancellation at each stage, the interference replica (desired signal) must be transferred between stages. The interference replica must be stored in a memory because a time difference is present between interference cancellation processes carried out for the same user at each stage. This interference replica is conventionally by a spread signal of each user and is represented by an oversampling value. This results in a necessity for a large-scale memory to store them.
The above interference replicas are represented by a composite signal of multipath signals. On performing a despreading process for the specific path signal, asynchronous path signal components of interference replica (desired signal) cause interference due to an autocorrelation. The influence of this interference cannot then be removed no matter how many stages are used in the receiving circuit.
A system has been proposed to solve this problem so that, on processing the specific path signal, interference signals for the other users are subtracted from input signal along with interference signals on other path signals of desired user. In other words, consideration has been made concerning the method in which the replica on only the specific path signal is added by an adder.
Such a technique has been described by Y. C. Yoon, R. Kohno, and H. Imai in a document entitled "A Spread-Spectrum Multi-Access System with a Cascade of Co-Channel Interference Cancellers for Multipath Fading Channels" (Proc. IEEE Second International Symposium on Spread Spectrum Techniques and Applications (ISSSTA) '92, pp. 87-90, December 1992).
In this document, all multipath signals other than the main path signal (the highest level path signal) are handled or regarded as interference. As a consequence, other multipath signals are not utilized to be combined into the desired signal. Stated otherwise, this is equivalent to using only the spread signal for the main path signal as the interference replica.
In contrast, if an interference replica is prepared for each path and addition and despread processes are made for each path, then during despread of the specific path signal, all multipath signals other than the specific path signal will not give any interference.
However this structure has the drawback that, since the spread signal for each path is utilized as the interference replica, a memory becomes very large.